In a detailed study, Felix Widdascheck, Alrun Hauke and Gregor Witte from SFB-project A2 show how phthalocyanine monolayers can be used to control the work function of noble metal electrodes, both in single crystalline model systems and for real life polycrystalline electrodes.

The work function of bare metal surfaces (yellow) can be modified by a thin layer of phthalocyanines (blue) to reduce injection barriers in organic electronic devices. (Image: F. Widdascheck).

Work function tailoring by means of organic monolayers is one of several promising approaches to reducing the contact resistance at the interface between metal electrodes and organic semiconductors in organic electronics devices.

In their study Felix Widdascheck and coauthors used several polar and non-polar phthalocyanines to modify the work functions of noble metal electrodes. As a starting point, they performed a detailed STM and Kelvin probe analysis of the coverage-dependent work function changes of Au and Ag single crystal surfaces. The authors find that the work function changes strongly depend on both coverage and the type of phthalocyanine used as the contact primer. Their phenomenological description of the observed trends provides important groundwork for more detailed theoretical modeling of the processes taking place at the complex internal interface between metal, monolayer and organic semiconductor.

In a further step towards actual device applications, the authors then transferred their findings and the developed preparation protocols to polycrystalline electrodes, demonstrating that the same work function changes can be observed also on “real-life” electrodes. With the end user in mind, the team also tested the air stability of their contact primers, proving that a sacrificial phthalocyanine multilayer serves well to protect the highly ordered mono- and bilayer contact primers during air transfer and can be removed by thermal desorption afterwards.

Publication

F. Widdascheck, A.A. Hauke and G. Witte,
A Solvent-Free Solution: Vacuum-Deposited Organic Monolayers Modify Work Functions of Noble Metal Electrodes
Adv. Funct. Mater. (2019) DOI: 10.1002/adfm.201808385

See also press release in German.

In organic electronic devices, such as modern displays with organic light-emitting diodes (OLEDs), organic materials connect to metal electrodes. The resulting metal–organic interfaces, which are in the focus of SFB 1083, determine important performance parameters such as rates of charge-carrier injection. Precise control over the interface properties, especially the wave-function overlap and the energy-level alignment, is therefore critical for rational improvement of organic electronic devices. Here, the SFB 1083 projects A4, A6 (both Univ. Marburg) and A12 (at FZ Jülich), together with groups in Utrecht (NL), Warwick (UK) and Erlangen (DE), show that the properties of metal-organic interfaces depend strongly on the linking pattern of the atoms in the organic material.

In organic semiconductors, the carbon atoms are typically laid out in a honeycomb-like sheet of abutting six-sided rings. If the sheet contains no odd-numbered rings, it is described as an “alternant topology.” Researchers rarely consider nonalternant topologies, which occur when the structure contains, for example, five- or seven-sided rings. To elucidate the influence of the topology on the interaction with a metal surface, the authors compare the aromatic hydrocarbon naphthalene to its nonalternant isomer, azulene, and study their interactions with a copper surface.

Azulene-like nonalternant 5-7 structural element embedded in a graphene lattice (right), compared to the ideal graphene lattice left. The figure shows sections through the charge density for both systems, according to DFT calculations. The 5-7 element accumulates negative charge (red) at the 5-membered ring and positive charge (blue/white) at the 7-membered ring. Copyright by CC-BY 4.0.

Benedikt Klein and his co-workers find that azulene forms a much stronger and shorter bond to copper than naphthalene. Spectroscopic analysis of the electronic structure reveals that azulene forms a true chemical bond and receives negative charge from the surface, whereas naphthalene bonds only weakly and does not exchange charge. Theoretical analysis reveals that the influence of the topology on the electronic structure, especially the lowest unoccupied molecular orbital, is responsible for the different behavior. This comprehensive analysis of a surface chemical bond was only possible through a multi-technique approach, which involved a collaboration between six research groups from experiment and theory, including three from SFB 1083. Important contributions were made by the groups of Ingmar Swart (Utrecht, NL), Reinhardt Maurer (Warwick, UK), and Wolfgang Hieringer (Erlangen, DE).

Based on their findings, the authors propose that the incorporation of nonalternant structural elements can be used to control and optimize performance-related properties of functional metal–organic interfaces.

Publication

B. P. Klein, N. J. van der Heijden, S. R. Kachel, M. Franke, C. K. Krug, K. K. Greulich, L. Ruppenthal, P. Müller, P. Rosenow, S. Parhizkar, F. C. Bocquet, M. Schmid, W. Hieringer, R. J. Maurer, R. Tonner, C. Kumpf, I. Swart, and J. M. Gottfried, Molecular topology and surface chemical bond: alternant versus nonalternant aromatic systems as functional structural elements, Physical Review X 9/1(2019) 011030 (17pp) DOI:10.1103/PhysRevX.9.011030

The meeting was jointly organized by SFBs 951 “Hybrid Inorganic/Organic Systems for Opto-Electronics (HIOS)” Berlin and SFB 1083 “Structure and Dynamics of Internal Interfaces”. It attracted more than 80 participants from Europe, Asia and America to the Black Forest. The meeting consisted of 30 talks and 48 posters.

The series of biannual ASOMEA-workshops began in 2001 as a meeting of Swedish and Japanese scientists working with spectroscopic techniques and theoretical modeling for a better understanding of organic electronic materials and related interfaces. In 2016 the scope of the workshop was widened to include the German community and the intention to focus on organic materials at advanced stages, in situ/operando techniques, and time-resolved spectroscopy to name just a few.

For more information visit https://asomea9.internal-interfaces.de/.

Johannes Reimann, Jens Güdde and Ulrich Höfer together with a team led by Rupert Huber in Regensburg have taken band structure movies of electrical currents carried by Dirac electrons as they are driven by an intense THz wave.

Artistic view of the experiment (image by Brad Braxley parttowhole.com).

The investigated currents consist of spin-polarized electrons confined to the uppermost atomic layers of the topological insulator Bi₂Te₃. The electrons were observed to react in an in­ertia-free fashion to the driving field, whereas spin-momentum locking lifts scattering times abo­ve 1 ps. This scenario enables giant sur­fa­ce cur­rent densities and bal­­listic mean free paths of several 100 nm, exceeding values obtained in conventional materials by or­ders of magni­tu­de. Based on this discovery, it might be possible to realize new lightwave-driven electronics in the future, combining low power consumption and clock rates that exceed those of conventional semiconductor devices by a factor of 1000 and more.

Animation of photoemission snapshots of the topological surface state in Bi₂Te₃ showing the back and forth acceleration of Dirac electrons at optical clock rates by an intensive Thz electric field.

The work of the two groups and collaborators in Novosibirsk and Hiroshima not only merges two novel and promising concepts in physics – topology and lightwave electronics. It also combines the expertise of the Regensburg group to manipulate electrons in solids with intense single-cycle terahertz (THz) transients, with the capabilities of time and angle-resolved photoelectron spectroscopy (ARPES) developed in Marburg.  The experiment conducted in Regensburg represents the first an­gle-re­­sol­ved pho­to­emis­sion spec­tro­scopy with subcycle resolution. It allowed Reimann and coworkers to directly ob­ser­ve how the car­rier wave of a te­rahertz pulse ac­celerates Di­rac fermions in the band­ struc­tu­­re. The resulting strong re­distribution in mo­men­tum space was directly mapped out in an ultrafast movie (Figure on larger screens).

In future experiments, it will be explored whether the topological protection responsible for the long scattering times at the Bi₂Te₃/vacuum interface survives when the material is covered by a protective cap layer as this is a prerequisite for device application.